PROCESS FOR PRODUCING A FORGED COMPONENT FROM A TiAl ALLOY AND COMPONENT PRODUCED THEREBY

ABSTRACT

A process for producing a component from a TiAl alloy by two-stage isothermal forging and to a component produced thereby. The process comprises a first isothermal forging of a component precursor at a temperature of at least about 1180° C., an intermediate annealing of the forged precursor at a temperature ranging from about 1130° C. to 1170° C. for about 1 to 8 hours and a subsequent second isothermal forging at a temperature of at least about 1180° C. with a degree of forming which is lower than the degree of forming in the first isothermal forging.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119 of German Patent Application No. 102018209881.6, filed Jun. 19, 2018, the entire disclosure of which is expressly incorporated by reference herein.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a process for producing a component from a TiAl alloy in which the component is formed by two-stage isothermal forging and is preferably subsequently subjected to a heat treatment. The present invention further relates to a correspondingly produced component.

2. Discussion of Background Information

TiAl alloys, the main constituents of which are titanium and aluminum, are notable in that they have a high strength, especially high-temperature strength, with sufficient ductility as a result of formation of intermetallic phases, for example γ-TiAl, which have a high proportion of covalent bonding forces within the metallic bond. In addition, they have a low specific weight, and therefore the use of titanium aluminides or of TiAl alloys is advantageous for high-temperature applications, for example for turbomachines, especially gas turbines or aircraft engines.

By adding certain alloying constituents, for example niobium and molybdenum, the property profile of the TiAl alloys can be further optimized. Alloys of this kind having niobium and molybdenum content are also referred to as “TNM alloys”.

These alloys are employed, for example, for producing stator vanes or rotor blades in aircraft engines and are brought into the appropriate component form by forging. In particular, it is possible here to use isothermal forging with subsequent heat treatment for adjusting the microstructure and the property profile. In this way, it is also possible to produce one-piece blade/disk units, referred to as blisks (portmanteau of blade and disk).

In the forging process, the forging temperature can be raised to 1200° C. and the main forming shifted from the second to the first forging step. As a result of this, the recovery mechanisms increase at the expense of the recrystallization mechanisms and a portion of the dislocation energy, which has to be used for refining the microstructure, is lost as a result of recovery. (Dislocation energy is that energy which is present as a result of small defects within the crystal lattice; dislocations are sandwiched planes of atoms that terminate in the middle of the crystal.) The dislocation density is increased by the forging to such an extent that this energy can be utilized for recrystallization processes. The microstructure is spheroidized in the process and as a result becomes finer and more homogeneous. This mechanism is the reason why forging is used in order to achieve higher strengths in materials. Due to an increasingly greater recovery of the microstructure, when recovery increases in relation to recrystallization as a result of raising the forging temperature and shifting the main forming from the second to the first forging step, the microstructure becomes more inhomogeneous and ductility or scatter in the total elongation increases. The strength potential and elongation potential of the material is as a result not exploited as fully as would be desirable for the application.

In view of the foregoing, it would be advantageous to have available a process for producing a component from a TiAl alloy in which the component is formed by two-stage isothermal forging at temperatures of approximately 1200° C. and is subsequently subjected to a heat treatment, but wherein the above described disadvantages are avoided.

SUMMARY OF THE INVENTION

The present invention provides a process for producing a component from a TiAl alloy by two-stage isothermal forging which comprises the following steps:

-   -   first isothermal forging of a component precursor at a         temperature of at least about 1180° C. and with a first degree         of forming;     -   intermediate annealing of the forged precursor at a temperature         in the range of from about 1130° C. to about 1170° C. for a         period of from 1 to 8 hours;     -   second isothermal forging of the intermediately annealed         precursor at a temperature of at least about 1180° C. and with a         second degree of forming which is lower than the first degree of         forming in the first isothermal forging.

The second degree of forming may for example be not more than about 95%, not more than about 90%, not more than about 85%, not more than about 80% or not more than about 75% of the first degree of forming.

In one embodiment of the process according to the invention, the TiAl alloy may especially be a TNM alloy, that is to say an alloy which also comprises, besides the main titanium and aluminum constituents, smaller proportions of niobium and molybdenum (and preferably also boron).

The process according to the invention may be employed particularly advantageously for components composed of a TiAl alloy having from about 42 at. % to about 45 at. % of titanium, especially from about 42.5 at. % to about 44.5 at. % of titanium, from about 3.5 at. % to about 4.5 at. % of niobium, especially from about 4.0 at. % to about 4.2 at. % of niobium, from about 0.75 at. % to about 1.5 at. % of molybdenum, especially from about 0.9 at. % to about 1.2 at. % of molybdenum, and from 0.05 at. % to about 0.15 at. % of boron, especially from about 0.1 at. % to about 0.12 at. % of boron, the remainder being aluminum and unavoidable impurities.

In one further embodiment of the process according to the invention, the temperature in the first and/or second forging step is at least about 1190° C., and especially at least about 1200° C., particular preference being given to a temperature of approximately 1200° C.

In one further configuration of the process according to the invention, the degree of forming in the first forging step in the inner region which comprises the final component geometry is at least about 0.55, e.g., at least about 0.6 or at least about 0.65.

In one further embodiment of the process according to the invention, the temperature in the intermediate annealing step is in the range of from about 1135° C. to about 1165° C., especially in the range from about 1140° C. to about 1160° C. Particular preference is given to a temperature in the range from about 1145° C. to about 1155° C. The hold time at the indicated temperature is at least about 1 hour, for example at least about 1.5 hours or at least about 2 hours, and is no longer than about 8 hours, for example no longer than about 7.5 hours or no longer than about 7 hours.

By introducing a one- to eight-hour intermediate annealing in the above temperature range between the isothermal forging steps, it is possible to raise the elongation potential and the strength of the TiAl material (for example a TNM alloy), especially when more local forming is brought about in the first isothermal forging step than in the second forging step. Furthermore, this lowers the flow stresses in the second isothermal forging step, as a result of which, as a rule, die service lives can be increased and die filling can be improved. This reduces die costs and increases and stabilizes the strength and the elongation potential of the component. The intermediate annealing may be performed for example in an oven.

The intermediate annealing is preferably followed (prior to the second isothermal forging step) by a cooling of the component precursor (preferably by means of air cooling). The component precursor can, however, also be raised directly up to the temperature for the second isothermal forging step without cooling.

In one further embodiment of the process according to the invention, the second isothermal forging step (to final contour or near net shape) is followed by at least one heat treatment of the forged component. This further heat treatment is preferably conducted at a temperature above the γ solvus temperature or below the γ solvus temperature, preferably for about 20 minutes to about 180 minutes in either case.

By way of example, the heat treatment is conducted at a temperature in the range from the γ solvus temperature to about 50° C., e.g., about 30° C., from about 2° C. to about 25° C., or from about 5° C. to about 25° C., above the γ solvus temperature, or at a temperature from about 10° C. to about 50° C., e.g., from about 15° C. to about 30° C., below the γ solvus temperature.

The further heat treatment is preferably followed by a cooling, preferably at a cooling rate of at least about 100° C./minute and not more than about 500° C./minute. The cooling may optionally be followed by a further heat treatment, for example at a temperature of about 800° C. to about 950° C. (stabilizing annealing).

By way of example, it is possible with the first isothermal forging step of the process according to the invention to bring about sufficient forming in, and thus introduce sufficient dislocation density into, the workpiece that, by means of a recrystallization annealing at, for example, about 1150° C. for about 1 to 8 hours with subsequent (preferably air) cooling, the microstructure is completely spheroidized to form globules to local degrees of forming of approximately 0.7. In the second isothermal forging step to final contour, this microstructure can be forged with little expenditure of force (a globular microstructure has little flow stress) into a near-net-shape form. Even though locally there is a lower degree of forming than is necessary for complete recrystallization (>0.7), by means of a subsequent heat treatment as described above, a homogeneous microstructure can be established without old residual colonies left behind from the cast state. This more homogeneous microstructure has a higher strength and thus a higher total elongation than components that were processed without this recrystallization annealing.

As a result of the above-described subsequent heat treatment for, by way of example, about 20 minutes to about 40 minutes (above the γ solvus temperature) or about 45 minutes to about 180 minutes in the three-phase field (below the γ solvus temperature) of the TiAl alloy, virtually lamellar microstructures tailored specifically to the requirements can be established. By means of a subsequent cooling at about 500° C./minute it is possible to establish very fine, creep-resistant lamellae, or by means of correspondingly slower cooling at down to about 100° C./minute thicker lamellae that are more resistant with respect to cellular reaction. Subsequently, by means of a precipitation hardening heat treatment for several hours, the phase fractions are adjusted to be close to the thermal equilibrium at application temperature.

Using the process according to the invention, for example, components of a turbomachine can be produced, especially components of a gas turbine or an aircraft engine, for example rotor blades, stator vanes or turbine blisks.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

A material for a component produced in accordance with the invention may, for example, have a composition in the range of from 42 at. % to 45 at. % of titanium, from 3.5 at. % to 4.5 at. % of niobium, from 0.75 at. % to 1.5 at. % of molybdenum, and also from 0.05 at. % to 0.15 at. % of boron, the remainder being aluminum and unavoidable impurities.

The material of the component is first forged by a first isothermal forging step at approximately 1200° C. with a degree of forming of approximately 0.6.

Next, the forged material is cooled to room temperature by means of air cooling and then heated at approximately 1150° C. for 4 hours, followed by a second isothermal forging step at approximately 1200° C. to (near) net shape.

The component thus produced can, in a first variant, be subjected to a (second) heat treatment at a temperature above the γ solvus temperature (for example at approximately 1290° C.) for, by way of example, 20 to 40 minutes.

Thereafter, the component is cooled rapidly, for example by means of fan cooling. This fan cooling is effected in air or in an oven, the temperature being lowered to below about 600° C. and then raised to approximately 850° C. and held at this temperature for approximately 6 hours.

The γ-TiAl microstructure that was established at the temperature of the second heat treatment, that is to say at a temperature of approximately 1290° C., is largely frozen in as a result.

In a second variant for the second heat treatment, the component is heated to below the y solvus temperature. By way of example, the component is heated at approximately 1235° C. for about an hour and subsequently cooled (fan cooling). The temperature is lowered to below 600° C. during the fan cooling and then raised again to approximately 850° C.

Although the present invention has been described in detail on the basis of the working example, it is clear to those skilled in the art that modifications are possible in such a way that individual features can be omitted, or different combinations of features can be implemented, without departing from the scope of protection of the claims. In particular, the disclosure of the present invention includes all combinations of the individual features presented above. 

What is claimed is:
 1. A process for producing a component from a TiAl alloy by two-stage isothermal forging, wherein the process comprises: (a) first isothermal forging of a component precursor at a temperature of at least 1180° C. and with a first total degree of forming; (b) intermediate annealing of the forged precursor of (a) at a temperature ranging from 1130° C. to 1170° C. for 1 to 8 hours; (c) second isothermal forging of the intermediately annealed precursor of (b) at a temperature of at least 1180° C. and with a second total degree of forming which is lower than the first total degree of forming.
 2. The process of claim 1, wherein the TiAl alloy is a TNM alloy.
 3. The process of claim 1, wherein the temperature in (a) is at least 1190° C.
 4. The process of claim 1, wherein the temperature in (a) is at least 1200° C.
 5. The process of claim 1, wherein the temperature in (c) is at least 1190° C.
 6. The process of claim 4, wherein the temperature in (c) is at least 1200° C.
 7. The process of claim 1, wherein the degree of forming in (a) is at least 0.55.
 8. The process of claim 4, wherein the degree of forming in (a) is at least 0.6.
 9. The process of claim 1, wherein the temperature in (b) ranges from 1135° C. to 1165° C.
 10. The process of claim 4, wherein the temperature in (b) ranges from 1140° C. to 1160° C.
 11. The process of claim 1, wherein (b) is conducted for at least 1.5 hours and/or for no longer than 7.5 hours.
 12. The process of claim 1, wherein after (c) at least one further heat treatment is conducted.
 13. The process of claim 12, wherein the further heat treatment is conducted at a temperature above a γ solvus temperature of the TiAl alloy.
 14. The process of claim 12, wherein the further heat treatment is conducted at a temperature below a γ solvus temperature of the TiAl alloy.
 15. The process of claim 12, wherein the further heat treatment is conducted at a temperature in the range from a γ solvus temperature to 30° C. above the γ solvus temperature of the TiAl alloy.
 16. The process of claim 12, wherein the further heat treatment is conducted at a temperature of from 10° C. to 50° C. below the γ solvus temperature of the TiAl alloy.
 17. The process of claim 12, wherein the further heat treatment is conducted for from 20 to 180 minutes.
 18. The process of claim 12, wherein the further heat treatment is followed by a cooling.
 19. The process of claim 18, wherein the cooling is followed by a further heat treatment.
 20. A component produced by the process of claim
 1. 